lazyancestors: reuse __iter__ implementation in __contains__
There was a comment in the code that said "Trying to do both __iter__
and __contains__ using the same visit heap and seen set is complex
enough that it slows down both. Keep them separate.". However, it
seems easy and efficient to make __contains__ keep an iterator across
calls.
I couldn't measure any slowdown from `hg bundle --all` (which seem to
call lazyancestors.__contains__ frequently).
Differential Revision: https://phab.mercurial-scm.org/D4508
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